Filed of Invention
The present invention relates to a power converter, and more particularly, the present invention relates to a switching control circuit of the power converter.
Description of Related Art
FIG. 1 shows a circuit diagram of a switching power converter. A switching control circuit 70 generates a switching signal SW in accordance with a feedback signal VFB. The feedback signal VFB is correlated to an output VO of the power converter. The switching signal SW is coupled to switch a transformer 10 of the power converter through a transistor 20. Thus, the transformer 10 will deliver the power from an input VIN of the power converter to the output VO of the power converter via a transistor 40 and a capacitor 50. The transformer 10 has a primary side and a secondary side. The input VIN is supplied to a primary winding NP at the primary side and the output VO is generated at the secondary side. The secondary side has a secondary winding NS. The transistor 40 is coupled between a first terminal of the secondary winding NS and an output terminal of the power converter. The capacitor 50 is coupled to a second terminal of the secondary winding NS and the output terminal of the power converter.
A rectifier 45 is parallel connected to the transistor 40. It can be a separated diode or a parasitic diode of the transistor 40. A SR controller 90 associates with the transistor 40 developed a synchronous rectifier (SR) at the secondary side of the transformer 10. Further, a switching current IP flows through the primary winding NP of the transformer 10 and the transistor 20 when the transistor 20 is turned on. A resistor 30 is coupled between the transistor 20 and a ground to detect the switching current IP of the transformer 10 for generating a current-sense signal VCS. The current-sense signal VCS is coupled to the switching control circuit 70.
Many prior arts had disclosed the skill of the synchronous rectifying, such as, “PWM controller for synchronous rectifier of flyback power converter” U.S. Pat. No. 6,995,991; “Synchronous rectification circuit for power converters” U.S. Pat. No. 7,440,298; “Method and apparatus for predicting discharge time of magnetic device for power converter” U.S. Pat. No. 7,511,466; “Offline synchronous rectifier with causal circuit for resonant switching power converter” U.S. Pat. No. 8,023,289; “Offline synchronous rectifier circuit with turned-on arbiter and phase-lock for switching power converters” U.S. Pat. No. 8,154,888.
Many controllers (e.g. the SR controller 90) of synchronous rectifying include the phase lock circuit to detect the switching waveform of the transformer 10 and predict the turn-on time of the SR transistor (such as the transistor 40). The response time of the phase lock circuit is limited, which restricts the maximum operation frequency and speed of the switching control circuit (e.g. the switching control circuit 70).
The present invention provides a signal process in the switching control circuit, which allows the switching control circuit can operate at high frequency without the limitation of the synchronous rectifier.